Pneumatic measurement devices are known that make use of a pressure measurement to determine the distance between a measurement nozzle of the device and a facing wall, which distance is itself representative of the size of the part to be inspected. In the most usual case, a pneumatic measurement device comprises a measurement chamber fed with gas under pressure, a measurement nozzle fed from the measurement chamber, and a pressure take-off disposed in the measurement chamber upstream from the nozzle. The pressure take-off is connected to a pressure gauge, thereby measuring the pressure in the measurement chamber, which pressure is a function of the leakage section between the nozzle and the wall of the part to be inspected, and thus of the distance between the wall and the nozzle.
More specifically, to measure the diameter of a bore of a part to be inspected, pneumatic measurement devices are known that are implemented in the form of plugs. Such pneumatic measurement plugs generally comprise an elongate cylindrical member having a pair of diametrically opposite measurement nozzles fed with gas under pressure via a measurement chamber that is fitted as before with a pressure take-off connected to a pressure gauge. The pressure measured by the gauge is a function of the distances between the wall of the bore and each of the nozzles. Since the distance between the ends of the nozzles is known, the measured diameter of the bore to be inspected can be deduced therefrom.
However, in practice, it is very difficult to cause the axis of the elongate member to coincide with the axis of the bore to be inspected, so the two measurement nozzles are generally situated at different distances from the wall of the bore. Unfortunately, the mathematical relationship relating the distance between the measurement nozzle and the wall with the pressure in the measurement chamber is not linear, such that the measured diameter of the bore varies depending on the position of the plug within the bore, and this gives rise to uncertainty on the real size of the bore. Furthermore, in order to enable the plug to be inserted inside the bore without rubbing against the wall of the bore even in the event of the axis of the measurement plug being offset relative to the axis of the bore, it is necessary to provide for the diameter of the measurement plug to be considerably smaller than the diameter of the bore to be measured, and that causes the distance between the measurement nozzles and the facing wall to be increased. Unfortunately, it is known that the accuracy with which measurement is performed decreases with increasing distance between the nozzle and the wall.
In order to remedy that drawback, pneumatic measurement plugs have been made in which the measurement nozzles are carried by a measurement ring that is itself mounted with radial play relative to the elongate member. This disposition makes it possible for the ring to be centered automatically in the bore even when the elongate member is offset relative to the axis of the bore, and thus to make rings that are very close in diameter to the diameter of the bore. However, the ring can be displaced radially on being inserted into the inside of the bore solely by said ring coming into contact with the bore. The end result is that the measurement ring is disposed off-center by the maximum possible amount within the bore and that tends to degrade measurement, in the manner explained above.